Rewrite transport

Looking for some feedback. This change would allow you to configure the [ajax request](http://api.jquery.com/jQuery.ajax/) so you could set headers, parse the response, etc. It should resolve #8, #13, #20.

My only concern is that this exposes too much. Rather than making every ajax option configurable, we could just whitelist the ones we think would be useful. Maybe exposing everything is the way to go though, after all who are we to deem which options are useful :smiley:.